Your good watering intentions may be the root problem (no pun intended). Arborvitaes are sensitive to soils that are too wet; they will start to turn yellow and eventually die when planted in a site with wet soil or if they are overwatered. The garden's soil is a heavy clay loam, for the most part, and tends to hold moisture. You need to take a closer look at the situation in your garden.
Arborvitae plants are fairly drought tolerant once they are established--too much water would be worse than dry conditions. While they like water, the soil needs to drain well. Clay soil is not known for its friability. It holds water far better than most soils, so this could be a factor.
The roots (especially of newly-planted) plants have a difficult time trying to spread through heavy, compacted clay soils. Hence they do not grow strong enough in the small amount of soil they were planted with to sustain health of the plant. Roots don't typically grow down very far, but they should spread far and wide. Your arborvitae likely has not been able to do this.
Considering the size of your arborvitae, it isn't likely you could amend the soil enough to save the tree. Your only chance may be to remove it.
If this particular tree is quite important to you, your best resource for diagnosing and treating your tree is to consult a certified arborist. An arborist is a certified and generally insured expert in trees and their problems. He or she can examine your tree and make recommendations regarding the health of the tree, any needed judicial pruning, and whether or not (in his opinion) it should be removed. You can find a certified arborist in your area here:
Click on "Find an Arborist" at the top of the page; then click on United States from the pull-down menu. Then enter your State and City. This should bring up a list of certified arborists nearest to you. Or you may be able to locate one under "Tree Services" in your local Yellow Pages.